1) Field of the Invention
The present invention relates to an optical device, adapted to be used when light is split into two orthogonal polarized light components contained therein, more particularly, adapted to be suitably used when light including a plurality of wavelengths is split into polarized light, and also relates to a polarization monitor and an optical switch.
2) Description of the Related Art
Conventionally, when polarized light of wavelength multiplexed (WDM: wavelength division multiplexing) light, is monitored, the light is split into each wavelength and thereafter its polarized light components are split by a polarizer, whereupon the polarized light is monitored. The polarizer is an element that splits and selects linearly polarized component of light from light having various polarizations.
As the polarizer, there can be enumerated e.g. a single image type polarizing prism, a double image type polarizing prism, and a birefringent plate, a polarized light beam splitter which utilize the Brewster's condition of multilayered dielectric thin films.
The single image type polarizing prism transmits only a component of orthogonal polarization, as Glan-Thompson prism or Nicol prism does. On the other hand, the double image type polarizing prism separate components of orthogonal polarization and output the separated components of orthogonal polarization, as Wallaston prism or Rochon Prism does.
The conventional polarizer is described in the literature “Optical Application Electronics Handbook”, first print of first edition written by Optical Application Electronics Handbook Editing Committee, 1989, published by SHOKODO Co., Ltd. and the literature “Optics” written by K. Ishiguro, 1977, published by KYORITSU SHUPPAN Co., Ltd.
When the polarized light of wavelength-multiplexed light is monitored, among the mentioned-above polarizers, the single image prism, the plural image prism, the birefringent plate, and multilayered dielectric thin film type polarized light beam splitter have been used.
FIGS. 31 and 32 are diagrams showing a conventional polarization monitor described in the Patent Document 1 mentioning below. At the polarization monitor described in the Patent Literature 1, a plane waveguide circuit is used, and after wavelength multiplexed light is de-multiplexed into for each wavelength, devices, which monitor the polarized light component for each wavelength, are integrated on one substrate.
Here, FIG. 31 is a block diagram showing a total structure of the polarization monitor, and a polarization monitor 201 shown in FIG. 31 has a structure that is provided with a wavelength de-multiplexing filter 210, polarized light dependent optical circuits 220a to 220d, and photoelectric conversion element arrays 230a to 230d. The wavelength de-multiplexing filter 210 de-multiplexes input light inputted from an input port 200 into each wavelength, and outputs the de-multiplexed input light as respective wavelength de-multiplexed light 202a to 202d to each of the polarized light dependent optical circuits 220a to 220d. 
And each of the polarized light dependent optical circuits 220a to 220d has a common structure as shown in the reference numeral 220 of FIG. 32, and also each of the photoelectric conversion element arrays 230a to 230d has a common structure as shown in the reference numeral 230 of FIG. 32. And as shown in FIGS. 31 and 32, by one to one relation between the polarized light dependent optical circuit 220 (220a to 220d) and the photoelectric conversion element array 230 (230a to 230d), a polarized state of an optical signal 202 (the reference numerals 202a to 202d in FIG. 31) of each wavelength de-multiplexed at the wavelength de-multiplexing filter 210 can be monitored.
Here, the polarized light dependent optical circuit 220 extracts four lights for measuring the polarized state of the optical signal 202 to which the wavelength de-multiplexing was applied at the wavelength de-multiplexing filter 210, and has a structure that is provided with beam splitters 203-1 to 203-3, polarizers 204-1 to 204-4, and mirrors 207-1 to 207-3. And the photoelectric conversion element array 230 has a structure that is provided with four photoelectric conversion elements 205-1 to 205-4 that receive four lights extracted at the polarized light dependent optical circuit 220 and outputs an electric signal corresponding to each of the intensity of the received light.
Actually, at each of the photoelectric conversion elements 205-1 to 205-3, output light from each of the polarizers 204-1 to 204-3 is inputted via each of the mirrors 207-1 to 207-3, and an electric signal corresponding to the intensity of the output light from each of the polarizers 204-1 to 204-3 is outputted. And at the photoelectric conversion elements 205-4, output light from the polarizers 204-4 is inputted, and an electric signal corresponding to the intensity of the output light from the polarizers 204-4 is outputted.
Therefore, at the technology described in the above-mentioned Patent Document 1, when the polarized state of light to be measured, in which a plurality of light is multiplexed, is measure, the polarized state of each wavelength is measured, after the light to be measured was split into each wavelength by the wavelength de-multiplexing filter.
[Patent Document 1] Japanese Patent Laid-Open (Kokai) 2003-107261
However, at the technology described in the above-mentioned Patent Document 1, in order to measure the polarized state of light in which a plurality of light is multiplexed, many polarized light splitting elements corresponding to the multiplexing number of light are required, with the result that there is a problem that the size of the polarization monitor becomes large.
And also there is a problem that the number thereof increases the cost of the polarized light splitting elements.
Moreover, there is a problem that the time and labor for adjusting the disposing positions of the polarized light splitting elements and manufacturing thereof are increased by the number of polarized light splitting elements.
Further, the de-multiplexing of wavelengths and the splitting of polarized light are executed separately, so that, there is a problem that the loss of light is increased.